Platelets and polymorphonuclear neutrophils (PMN) are important effector cells of the hemostatic and inflammatory responses to tissue injury. The dynamics of interaction between platelets and PMN provides a basis for understanding relevant pathophysiologies. Studies in vitro on platelet-PMN interactions have revealed both stimulatory and inhibitory effects on PMN function. We are investigating the intercellular mechanisms of platelet-PMN interactions and the functional conequences of platelet-PMN adhesion. Platelets and PMN were isolated from autologous human blood. Migrated and non-migrated PMN were separated after N-formylmethionyl-leucyl-phenylalanine (FMLP) activation. Platelets were labeled with a fluorescent monoclonal antibody directed against CD41. Platelets (300 million/ml) and PMN (3 million/ml) were incubated together. Heterotypic cell adhesion was measured in isolated PMN and PMN co-incubated with platelets by flow cytometric analysis of platelet marker fluorescence in PMN gated events. Platelet-PMN adhesion was also visualized by fluorescence microscopy. In studies of isolated PMN, contaminating platelets were bound to 16-34% of unstimulated PMN, 7-22% of stimulated PMN, 2-4% of migrated PMN, and 17-24% of non-migrated PMN. When platelets were co-incubated with migrated or non-migrated PMN, 15-78% of PMN bound one or two platelets. A preliminary study showed that platelet-PMN co-incubation did not affect the surface expression of adhesion molecules CD11a, CD11b, CD 41 and CD 44 on FMLP-stimulated platelets. Previous studies have shown that their expression is down-regulated differentially on migrated PMN. Our present studies show that unactivated platelets adhere to isolated PMN in vitro. Fewer platelets were adhered to migrated PMN than to non-migraterd PMN in isolated PMN preparations. These results indicate that platelets adhering to PMN are removed during PMN migration.